This invention relates to a temperature compensating circuit for a photographic exposure meter.
At the present time photographic exposure meters employing a silicon photo-diode as the light sensing element are extensively used. In exposure meters of this type of photoelectric current i.sub.P of the light sensing element varies with respect to the apex (Additive Photographic EXposure system) value B.sub.v of the brightness of an object to be photographed in accordance with the following equation: EQU i.sub.P = i.sub.PO .multidot.2.sup.B.sbsp. ( 1)
where i.sub.PO is the photoelectric current when B.sub.v = 0. It is difficult to directly indicate such a logarithmically varying photoelectric current with a conventional meter, however.
In order to overcome this difficulty a circuit has been proposed in which the photoelectric current is supplied to a logarithmic compression circuit utilizing the base-emitter voltage vs. collector current characteristic of a transistor or a diode, to thereby linearize the output and facilitate a more readable indication. The base-emitter voltage of the transistor or the diode characteristic varies with temperature at a rate of approximately -2mV/.degree. C, however, and in order to compensate for this temperature characteristic intricate and costly circuitry has been required.